Controlling ground-sourced water vapor is critical for long-term performance of basements, slabs, and foundation walls. Learn about vapor retarders, permeance levels, and compatible membrane systems.
Overview
Below-grade construction is constantly exposed to moisture vapor from the surrounding soil. When vapor barriers or retarders are not properly designed, this vapor can enter the building, increasing humidity, damaging finishes, and degrading indoor air quality. This article examines vapor control strategies for slabs, crawl spaces, and foundation walls, including product options and performance guidance.
Where Vapor Retarders Are Used
- Under slab-on-grade construction
- Over gravel beds and drainage layers
- On the warm side of basement insulation
- In crawl spaces or unvented foundation spaces
Performance Criteria
- Perm rating: Class I (< 0.1 perms) or Class II (0.1–1.0 perms)
- Durability to resist punctures and construction traffic
- Overlaps and taped seams to create continuity
- Compatibility with adhesives, tapes, and insulation
- Complies with ASTM E1745 (under-slab vapor retarder standard)
🔍 Quick Picks
- Best Under-Slab Vapor Barrier: Stego Wrap Class A 15-Mil Vapor Barrier
- Best Vapor Tape: Stego Tape 180
- Best Crawl Space Liner: Americover 12-mil Crawl Space Vapor Retarder
🛒 Product Highlights
- Stego Wrap 15-Mil Vapor Barrier: A Class A below-slab vapor retarder with puncture resistance and low perm rating.
- Stego Tape 180: Heavy-duty, pressure-sensitive tape for sealing overlaps in vapor retarder systems.
- Americover Crawl Space Liner: Reinforced, durable 12-mil vapor barrier ideal for use over soil or gravel in unvented spaces.
🧱 Used In These Assemblies
- Slab-on-grade commercial construction
- Basement wall waterproofing with interior insulation
- Sealed crawl space encapsulation projects
❓ FAQ
What’s the difference between a vapor retarder and vapor barrier?
They refer to materials that slow or stop water vapor diffusion. A “barrier” is typically Class I (most resistant).
Should I install vapor barrier under slab insulation?
Yes. Always place vapor barrier below insulation to protect it and prevent condensation within the slab.
🛠️ Related Articles
- Waterproofing Layers Below Grade Explained
- Insulating Basement and Foundation Walls
- Moisture Barrier vs Vapor Barrier: What’s the Difference?
Conclusion
Proper vapor control in below-grade construction protects indoor air quality and reduces risk of condensation damage. Choose a tested barrier product with proper thickness and install it with attention to continuity, seam sealing, and long-term durability.
Disclosure: As an Amazon Associate, EnvelopeIQ earns from qualifying purchases.
